On Wednesday Frazer said machete-wielding gangs had burnt homes and businesses in the western province, and were trying to force out the Kikuyu people, the tribe of Mwai Kibaki, the president.

McCormack said much of the violence in Kenya's Rift Valley was the result of "political tensions".

"There's a serious issue of people being displaced for a variety of different reasons, including being forced from their homes based on ethnic identification," he said, adding that the department's war crimes office was reviewing the incidents.

Documenting atrocities

"If they do document any instances of atrocities, we'll have to look at what next steps to take, but at this point we're not there yet," McCormack said.

"Very often, the case with these kind of circumstances is that you don't have a full understanding, a complete picture of what happened until after the situation is over and things have calmed down," he said.

Frazer said that the US wanted an investigation into the violence, including the killing of civilians by police, and said guilty parties must be held accountable.

"There has been an organised effort to push out people from Rift Valley ... It is clearly ethnic cleansing. I don't consider it genocide," she said.

"The cycle of retaliation has gone too far and has become more dangerous."

Shoot-to-kill orders

Her remarks followed AFP news agency reports that Kenyan police had been given "shoot-to-kill" orders in an attempt to stop the unrest sparked by Kibaki's disputed re-election.

"We have orders to shoot to kill these categories of people if they are caught in the act"

Kenyan police commander

"We have orders to shoot to kill these categories of people if they are caught in the act."

Soldiers armed with assault rifles and whips patrolled the streets of Naivasha in the Rift Valley on Wednesday, a day after military helicopters firing rubber bullets were used to disperse angry crowds.

"Police will henceforth be very forceful on groups of persons carrying out activities that threaten the lives and property of others," Eric Kraithe, Kenya's police spokesman, said.

Al Jazeera's Yvonne Ndege, reporting from Nairobi, said that police officials had told her that there was no explicit shoot-to-kill order, but the law allowed police officers to shoot on sight any people threatening life or property.

It would have been the second time since the wave of violence began after the December 27 vote that police have been told to shoot to kill.

Similar orders were given in January when police officers came under attack from gangs.

Ongoing talks

Talks between Kibaki and Raila Odinga, the opposition leader who accuses the president of rigging the election, began in the capital on Tuesday with both men calling for calm.

The two are meeting again on Thursday in the talks brokered by Kofi Annan, the former UN secretary-general.

In the first days after the election, the Kikuyu suffered much of the violence at the hands of Odinga's Luo tribe and other minority groups, but has since carried out numerous revenge attacks.

Leaders warned

Separately, Francis Deng, the UN special adviser on preventing genocide and mass atrocities, has warned that leaders responsible for the post-election violence could be held accountable for violations of international law.

Deng specified that he was not saying that anything that had happened so far in Kenya amounted to genocide.

"We're not talking the G-word at this point, but the kind of atrocities we're seeing could easily escalate to dangerous levels," he said.

Meanwhile, Amnesty International (AI), the London-based rights group, called for the protection of several Kenyan human rights defenders and activists who have received death threats.

The group, which includes six men and three women, have received a number of anonymous threats in the form of SMS messages, phone calls and emails.

They are now taking precautions for their safety, such as moving house and not making any public statements.

The threats include accusations that they are "traitors" to their ethnicity.

About 8,000 displaced people remain in a police compound in the town where they have been sheltering since violence erupted there several days ago.